A microfluidic system of this type, known from EP 1 217 099, forms a microreactor in which the microchannel structure for the fluids to be mixed (educts) and the reaction product are fashioned in a first plate and the further channel structure for the heat transfer fluid is fashioned in a further plate. The two plates are joined with a third plate, which forms a separating wall between the microchannel structure and the further channel structure, inset between them. The heat transfer fluid is temperature-controlled, i.e. heated or cooled, externally.